To a predator swimming below, a squid looks like a dark, delicious morsel. To blend in with the moonlit waters, some squid use glowing bacteria to erase their shadow. But how do squid collect these rare microbes? Researchers now report that squid cast nets to capture their symbiotic partners. This mechanism, other researchers say, could provide clues to human lung diseases.
Many organisms strike up cooperative relationships with bacteria, taking up their biochemical services. The squid Euprymna scolopes hosts bacteria known as Vibrio fischeri, which pump out light for camouflage. It's a successful guise, but one not easily acquired. Newly hatched squid have to harvest the rare bacteria from seawater. The squid uses tiny hairs to sweep seawater by the entrance to its light organ, but given the scarcity of the glowing bacteria, it's unlikely they find their way into the organ by chance.
Intrigued, a team of researchers led by Margaret McFall-Ngai of the University of Hawaii's Kewalo Marine Laboratory watched the process with the help of modified bacteria that produce a fluorescent protein so they could track individual bacteria under the microscope. To their surprise, when they put squid in seawater containing Vibrio, the squid cast a mucouslike net. After thousands of bacteria land on the net, they migrate into the light organ, where they form a colony and produce the light that protects the squid, the researchers report in the 29 August issue of the Proceedings of the National Academy of Sciences.
The strategy is similar to one used by plants to gather nitrogen-fixing bacteria, says team member Ned Ruby of the University of Hawaii. Microbiologist Peter Greenberg of the University of Iowa in Iowa City points out that human lungs capture and dispose of unwanted bacteria in much the same way. Greenberg hopes that further details of how the squid casts its net will "give us clues to treating human lung infections."